printf unification: floating point.

The only remaining differences between vfprintf.cpp and vfwprintf.cpp
after this are the wide/narrow conversions for %c, %m, and %s. I've used
"chars" and "bytes" for the named constants for the directions because
(a) I find -1 and 1 pretty confusing and (b) although "narrow" is the
obvious opposite of "wide", only Windows actually moved to wide
characters, so "narrow" (aka "multibyte", and probably "utf8") is the
default/normal case. Even though C confuses bytes and characters via its
`char` type, "bytes" versus "chars" seems like the appropriate
terminology (and it's what Java/Python use).

Also improve the swprintf tests assertion so failures are readable.

Test: treehugger
Change-Id: Ife8f70f65ec28d96058a7d68df353945524835d2
diff --git a/libc/stdio/vfwprintf.cpp b/libc/stdio/vfwprintf.cpp
index 2f86b3c..d6f6a6b 100644
--- a/libc/stdio/vfwprintf.cpp
+++ b/libc/stdio/vfwprintf.cpp
@@ -39,7 +39,7 @@
 #define CHAR_TYPE_inf L"inf"
 #define CHAR_TYPE_NAN L"NAN"
 #define CHAR_TYPE_nan L"nan"
-#define CHAR_TYPE_ORIENTATION 1
+#define CHAR_TYPE_ORIENTATION ORIENT_CHARS
 
 #define PRINT(ptr, len)                                          \
   do {                                                           \
@@ -355,10 +355,6 @@
         }
         if (prec < 0) prec = dtoaend - dtoaresult;
         if (expt == INT_MAX) ox[1] = '\0';
-        free(convbuf);
-        cp = convbuf = helpers::mbsconv(dtoaresult, -1);
-        if (cp == nullptr) goto error;
-        ndig = dtoaend - dtoaresult;
         goto fp_common;
       case 'e':
       case 'E':
@@ -395,11 +391,14 @@
           }
           if (expt == 9999) expt = INT_MAX;
         }
+      fp_common:
+#if CHAR_TYPE_ORIENTATION == ORIENT_BYTES
+        cp = dtoaresult;
+#else
         free(convbuf);
         cp = convbuf = helpers::mbsconv(dtoaresult, -1);
         if (cp == nullptr) goto error;
-        ndig = dtoaend - dtoaresult;
-      fp_common:
+#endif
         if (signflag) sign = '-';
         if (expt == INT_MAX) { /* inf or nan */
           if (*cp == 'N') {
@@ -412,6 +411,7 @@
           break;
         }
         flags |= FPT;
+        ndig = dtoaend - dtoaresult;
         if (ch == 'g' || ch == 'G') {
           if (expt > -4 && expt <= prec) {
             /* Make %[gG] smell like %[fF] */
@@ -652,6 +652,7 @@
     } else { /* glue together f_p fragments */
       if (decimal_point == nullptr) decimal_point = nl_langinfo(RADIXCHAR);
       if (!expchar) { /* %[fF] or sufficiently short %[gG] */
+        CHAR_TYPE* end = cp + ndig;
         if (expt <= 0) {
           PRINT(zeroes, 1);
           if (prec || flags & ALT) PRINT(decimal_point, 1);
@@ -659,11 +660,11 @@
           /* already handled initial 0's */
           prec += expt;
         } else {
-          PRINTANDPAD(cp, convbuf + ndig, lead, zeroes);
+          PRINTANDPAD(cp, end, lead, zeroes);
           cp += lead;
           if (prec || flags & ALT) PRINT(decimal_point, 1);
         }
-        PRINTANDPAD(cp, convbuf + ndig, prec, zeroes);
+        PRINTANDPAD(cp, end, prec, zeroes);
       } else { /* %[eE] or sufficiently long %[gG] */
         if (prec > 1 || flags & ALT) {
           buf[0] = *cp++;